First application of core-shell Ag@Ni magnetic nanocatalyst for transfer hydrogenation reactions of aromatic nitro and carbonyl compounds

Article abstract of DOI:10.1039/c2ra22143h

A magnetically separable core-shell Ag@Ni nanocatalyst was prepared by a simple one-pot synthetic route using oleylamine both as solvent and reducing agent and triphenylphosphine as surfactant. The synthesized nanoparticles were characterized by several techniques such as X-ray diffraction pattern (XRD), high resolution transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED) pattern, and energy dispersive X-ray spectroscopy (EDS). The core-shell Ag@Ni nanocatalyst was found to have very excellent activity for the transfer hydrogenation reactions of aromatic nitro and carbonyl compounds under mild conditions using isopropyl alcohol as hydrogen donor. Excellent chemoselectivity and regioselectivity for the nitro group reduction was demonstrated. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ra22143h

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First application of core-shell Ag@Ni magnetic
nanocatalyst for transfer hydrogenation reactions of
Cite this: RSC Advances, 2013, 3, 1050
aromatic nitro and carbonyl compounds3
Manoj B. Gawande,*a Huizhang Guo,b Anuj K. Rathi,c Paula S. Branco,a
Yuanzhi Chen,b Rajender S. Varmad and Dong-Liang Peng*b
Received 12th September 2012,
Accepted 15th November 2012
DOI: 10.1039/c2ra22143h
www.rsc.org/advances
A magnetically separable core-shell Ag@Ni nanocatalyst was
prepared by a simple one-pot synthetic route using oleylamine
both as solvent and reducing agent and triphenylphosphine as
surfactant. The synthesized nanoparticles were characterized by
several techniques such as X-ray diffraction pattern (XRD), high
resolution transmission electron microscopy (HR-TEM), selected
area electron diffraction (SAED) pattern, and energy dispersive
X-ray spectroscopy (EDS). The core-shell Ag@Ni nanocatalyst was
found to have very excellent activity for the transfer hydrogena-
tion reactions of aromatic nitro and carbonyl compounds under
mild conditions using isopropyl alcohol as hydrogen donor.
Excellent chemoselectivity and regioselectivity for the nitro group
reduction was demonstrated.
nanocatalyst,5 Pd–Pt core-shell nanowire catalysts for the oxygen
reduction reaction,6 Pd metal-containing core-shell catalysts for
oxidation reaction,7 and Ag–CeO2 core-shell nanocatalyst for
reduction reactions,8 etc.
Magnetically separable nanocatalysts is an important class of
the core-shell nanocatalyst family. In recent years, they have been
used in various important organic transformations because they
are robust, air stable, avoid traditional filtration processes, are
reusable, and are recyclable for several reaction runs.
These important features make them versatile, and inexpensive
heterogeneous catalysts. However, the scope of magnetically
separable core-shell nanocatalysts is largely untapped. There are
only some reports on magnetically separable core-shell nanocata-
lysts, such as Fe@Pt core-shell structures used as electrocatalysts
for ammonia borane oxidation,9 mesoporous-silica-protected core-
satellite nanocomposite catalysts,10 core-shell magnetic gold
nanocatalyst Fe3O4@MgAl–LDH@Au for oxidation reactions,11
dendron-functionalized core-shell nanocatalyst for C–C coupling
reactions,12 core-shell nanoparticle-supported TEMPO catalyst for
oxidation of alcohols,13 Au@Co core-shell nanocatalyst for
dehydrogenation of ammonia borane,14 and some other impor-
tant core-shell nanocatalyst.15
Introduction
The development and design of core-shell nanomaterials has
emerged in recent years because of their multifaceted applications
utilizing their electronic, magnetic, and optical properties and as
catalysts for organic reactions.1 These bimetallic core-shell
nanomaterials have gained much attention due to their distinctive
properties and impending scope in multiple fields. There are
various reports on core-shell nanomaterials and their applications
in catalysis, such as core-shell Au@SnO2 used for CO oxidation,2
thermally stable Pt/mesoporous silica core-shell nanocatalyst for
high-temperature reactions,3 Pd/FePt nanoparticles for reduction
reactions,4 gold-containing core-shell catalyst for a fuel cell
In continuation of our efforts for the development of greener
protocols, heterogeneous catalysis16 and nanomaterials and their
applications,17 a facile, simple, and inexpensive Ag@Ni core-shell
magnetically separable catalyst is presented for hydrogen transfer
reactions using IPA (isopropyl alcohol) as hydrogen donor. Very
recently, Ag@Ni core-shell nanocatalyst have been designed by
simple one-pot synthetic route.18 (Scheme 1, ESI ). To the best of
3
our knowledge this is the first report of Ag@Ni core-shell
magnetically separable nanocatalyst for hydrogen transfer reac-
tions of aromatic nitro and carbonyl compounds.
The synthesized nanoparticles (NPs) were characterized by
several techniques, such as X-ray diffraction pattern (XRD), high
resolution transmission electron microscopy (HR-TEM), selected
area electron diffraction (SAED) pattern, and energy dispersive
a
´
ˆ
REQUIMTE, Departamento de Quımica, Faculdade de Ciencias e Tecnologia, FCT,
Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
E-mail: mbgawande@yahoo.co.in; m.gawande@fct.unl.pt; Fax: +351 21 2948550;
Tel: +351 21 2948300
bDepartment of Materials Science and Engineering, College of Materials, Xiamen
University, Xiamen 361005, People’s Republic of China. E-mail: dlpeng@xmu.edu.cn
cJubilant Chemsys Ltd., B-34, Sector-58, Noida-201301, New Delhi, India
dSustainable Technology Division, National Risk Management Research Laboratory,
US Environmental Protection Agency, MS 443, 26 West Martin Luther King Drive,
Cincinnati, Ohio, 45268, USA
X-ray spectroscopy (EDS) (ESI ).
3
Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra22143h
3
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1.78 Å corresponding to the (200) planes of Ni can be observed. In
order to further confirm the formation of core-shell structure,
high-angle annular dark-field (HAADF) imaging along with EDS
line scan performed using scanning TEM (STEM) was used to
characterize the distribution of elements in single NPs. The
compositional scan profile (red line marked in the HAADF image
in Fig. 1d) on a single particle clearly shows strong Ni signals at
the particle edge and strong Ag signals in the particle center,
which reveals a typical core-shell structure. Both HRTEM and line
scan analysis results give convincing proof of the formation of Ag–
Ni core-shell structure. The synthesized Ag@Ni core-shell NPs are
equipped with a spherical core with a diameter of about 8.1 nm
and shell with a thickness of about 3.3 nm.
Scheme 1 Synthesis of Ag@Ni core-shell magnetic nanocatalyst.
Results and discussion
The Ag@Ni core-shell nanoparticles were explored as a catalyst
for the hydrogen transfer reactions of aromatic nitro compounds
(1) using isopropyl alcohol (IPA) as the hydrogen donor (Table 1).
First, catalytic activity of Ag@Ni was tested for the reduction of
4-hydroxy nitrobenzene (1i) to yield 4-hydroxyaniline (2i) under
various reaction conditions (Scheme 2). After optimization of the
The TEM image of Ag@Ni core-shell NPs shows a spherical
morphology with a diameter of 14.9 nm (Fig. 1a). These NPs have
a very narrow size distribution with a standard deviation of 1.14
nm. The selected area electron diffraction (SAED) pattern (Fig. 1b)
acquired from these NPs displays diffraction rings which can be
indexed to both face-centered cubic (fcc) Ni and Ag, respectively.
The XRD patterns conform well with SAED analysis. The SAED and
XRD analysis results indicate that Ni and Ag exhibit as two
separated crystalline phases instead of an alloy or intermetallic in
the products.18 This is because Ni and Ag are phase separated at
room temperature, which is very important to maintain the
catalytic property of these NPs. To characterize the core-shell
structure of the as-synthesized NPs, high-resolution TEM
(HRTEM) was conducted on a single nanoparticle. As shown in
Fig. 1c in the center area of a single NP, clear lattice fringes with a
spacing of 2.33 Å corresponding to the (111) planes of Ag can be
observed, while in the edge area lattice fringes with a spacing of
reaction conditions (ESI ), the catalytic activity of Ag@Ni core-shell
3
MNPs with other nitro substrates was further explored (Table 1).
The catalytic performance of Ag@Ni core-shell nanocatalyst,
was not significantly influenced by the nature and position of the
substituents on the aromatic nitro compounds. Electron-donating
as well as -withdrawing groups such as –Cl, –F, –Br, and –OCH3
did not have significant influence on the reaction (Table 1).
Notably, transfer hydrogenation over Ag@Ni core-shell nanocata-
lysts exhibits excellent activity and selectivity for the hydrogenation
of a series of halogenated nitrobenzenes to the corresponding
halogenated amines, without any dehalogenation (Table 1, entries
2c to 2g). Ag@Ni core-shell magnetic nanocatalysts also show
excellent regio- and chemoselectivity (Table 1, entries 2l to 2o). All
the nitro arenes were reduced in excellent yields affording a single
product, which minimizes the efforts to separate unreacted
starting compounds.
To further explore the versatility of the catalyst, the catalytic
performance was tested for the hydrogen transfer reactions of
carbonyl compounds 3 (Scheme 3). The general reaction proved to
be widely applicable as the corresponding alcohols were obtained
in good to excellent yields (Table 2, yield 90–94%). Interestingly,
no dehalogenation of the halogen derivatives of acetophenones
was observed (Table 2, entries 4b and 4c). Cyclohexanone was
successfully converted to cyclohexanol in good yield (Table 2, entry
4d).
A proposed mechanism for the reduction of aromatic nitro
compounds is depicted in Scheme 4. As a result of the adsorption
of IPA on the NPs surface, a hydride is transferred to the nitro
group with concomitant formation of acetone.17c Meanwhile, the
rate of reaction can show dependence on the strength of
adsorption of both isopropanol and the nitro compound. The
nitroso intermediate III, formed on elimination of water, adsorbed
on the Ag@Ni core-shell nanocatalyst, is again reduced by the
hydride transfer from IPA to the hydroxylamine IV which is further
reduced to the aniline V.
Fig. 1 Ag–Ni core-shell nanoparticles. (a) Low-magnification TEM image (at 50 nm);
(b) SAED pattern; (c) HRTEM image of a single nanoparticle; (d) EDS spectra
recorded from the center.
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Table 1 Ag@Ni core-shell catalyzed hydrogen transfer reactions of aromatic
nitroarenesa
2
Nitro compound Amines
Time (h) Isolated yield (%)
2a
2
96
Scheme 2 Hydrogen transfer reactions of aromatic nitro compounds on Ag@Ni
2b
2c
2.5
91
core-shell nanocatalyst.
3
86
A more effective and competitive interaction between the nitro
group and the NPs can justify the chemoselectivity observed for
the substrates 2m–o.
In heterogeneous catalysis, the stability of the catalyst is an
important issue from the viewpoint of a green sustainable
protocol. So, keeping this in mind, the stability of Ag@Ni core
shell magnetic nanoparticles was tested by recycling and reusing
the catalyst eight times without any significant loss of the catalytic
activity. After each cycle Ag@Ni MNPs were separated using an
external magnet and used for the next cycle (Fig. 2).
To check the efficiency of this protocol, the reduction of
nitrobenzene to aniline was compared to some existing protocol.19
In order to prove the reaction is heterogeneous, a standard
leaching experiment was conducted by the hot filtration method.
The model reaction of the reduction of 4-hydroxynitrobenzene
proceeded for 20 min in the presence of Ag@Ni at 80 uC. The hot
filtered reaction mixture was then stirred without catalyst for 12 h.
Notably, no formation of product was observed even after 12 h
indicating that no homogeneous catalyst was involved. Inductively
coupled plasma atomic emission spectra (ICP-AES) analysis of the
filtrate (hot) revealed the absence of Ag and Ni species in the
filtrate.
2d
2e
2.5
2.5
94
93
2f
3
3
89
88
2g
2h
2
93
2i
2j
2
3
90
85
A facile, effective, and magnetic core-shell nanocatalyst has
been designed for hydrogen transfer reactions. The reaction was
broadly applicable as diverse aromatic nitro and carbonyl
compounds were successively converted to the corresponding
amines or alcohols in excellent yield (85–94%) and short reaction
times. Being magnetically separable, this core-shell catalyst is a
promising candidate for other important organic conversions and
industrial applications.
2k
2l
2
2
93
94
2m
2n
2
2
91
93
Experimental
All commercial reagents were used as received unless otherwise
mentioned. For analytical and preparative thin-layer chromato-
2o
2.5
94
a
Reaction conditions: nitroarene (1 mmol), KOH (1.5 mmol), 80 uC,
IPA (3 mL), 50 mg of catalyst.
Scheme 3 Ag@Ni core-shell catalyst for hydrogenation of carbonyl compounds
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Table 2 Ag@Ni core-shell catalyzed hydrogen transfer reactions of carbonyl
compoundsa
4
Carbonyl compound
Products
Time
2
Yield (%)
94
4a
4b
4c
4d
2.5
2.5
2
92
90
90
Fig. 2 Reusability of Ag@Ni nanocatalyst (inset: catalyst separated by magnet).
1
mm probe at 300 MHz. H shifts are reported relative to internal
TMS.
Catalyst preparation
a
Reaction conditions: carbonyl compound (1 mmol), KOH (1.5
Ag–Ni core-shell nanoparticles were prepared via the thermal
decomposition of nickel(acetylacetonate)2 (Ni(acac)2, 96%, Acros)
and AgNO3 (99.8%, SCRC) in oleylamine (80–90%, Acros) using a
one-pot seed-growth method. In a typical synthesis, a mixture of 6
ml of oleylamine, 0.1 mmol of triphenylphosphine (TPP; CP grade,
SCRC), 0.4 mmol of Ni(acac)2, and 0.1 mmol of AgNO3 was
decanted into a three necked flask and kept under a flow of high-
purity argon gas at room temperature for 20 min. After that, the
mixture was heated to 80 uC and kept at this temperature for 15
min with strong magnetic stirring. The resulting solution was then
slowly heated up to a temperature of 190 uC (the reaction
temperature may range from 180 to 230 uC) directly and aged for
40 min. After cooling down to room temperature naturally, excess
acetone was added to the black solution to give a black precipitate,
which was isolated via centrifugation. The precipitate was then
washed thoroughly with a mixture of hexane and acetone and
dried in a vacuum.
mmol), 80 uC, IPA (3 mL), 50 mg of catalyst.
graphy, Merck 0.2 mm and 0.5 mm Kieselgel GF 254 percoated
plates were used, respectively. The spots were visualized using UV
light.
X-ray diffraction (XRD) patterns of the synthesized nanoparti-
cles were recorded using
a PANalytical X’pert PRO X-ray
diffractometer with Cu-Ka radiation. Transmission electron
microscopy (TEM) was performed on a TECNAI F-30 transmission
electron microscope operating at 300 kV. Energy dispersive X-ray
spectroscopy (EDS) analyses in both spot and line-scan mode were
used to identify the chemical components of single nanoparticles.
The TEM samples were prepared by dropping the particle
suspensions in toluene onto a copper grid coated with carbon
film before drying at room temperature under ambient conditions.
Elemental analysis was performed by using ICP-AES using a
Horiba Jobin-Yvon, France, Ultima model equipped with a 40.68
MHz RF generator, Czerny–Turner monochromator with 1.00 m
(sequential), auto sampler AS500 and CMA (concomitant metals
analyzer). Proton NMR spectra were recorded on a Bruker, 300, 5
General method for the reduction of nitroarenes and carbonyl
compounds
The nitroarene 1 or the carbonyl compound 3 (1 mmol), isopropyl
alcohol (3 mL), KOH (1.5 mmol), and Ag@Ni (50 mg), were stirred
at 80 uC for an appropriate time. After completion of the reaction
(monitored by TLC), the catalyst was separated magnetically. The
ensuing product was extracted with ethyl acetate and repeatedly
washed with water (5 to 7 times) to remove KOH. Then, the
organic solvent was evaporated in vacuo. The crude product was
purified by column chromatography on silica gel using n-hexane
and ethyl acetate as eluent.
Acknowledgements
˜
ˆ
This work has been supported by Fundaçao para a Ciencia e a
Tecnologia through grant no. PEst-C/EQB/LA0006/2011., the
National Outstanding Youth Science Foundation of China
(Grant no. 50825101), and the National Natural Science
Foundation of China (Grant no. 50971108 and 51171158).
Manoj B. Gawande thank PRAXIS program for the award of
research fellowship SFRH/BPD/64934/2009.
Scheme 4 Possible mechanism for hydrogen transfer reactions over Ag@Ni
nanocatalysts.
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8 T. Mitsudome, Y. Mikami, M. Matoba, T. Mizugaki,
K. Jitsukawa and K. Kaneda, Angew. Chem., Int. Ed., 2012, 51,
136–139.
9 X. B. Zhang, J. M. Yan, S. Han, H. Shioyama and Q. Xu, J. Am.
Chem. Soc., 2009, 131, 2778–2779.
10 J. P. Ge, Q. Zhang, T. R. Zhang and Y. D. Yin, Angew. Chem., Int.
Ed., 2008, 47, 8924–8928.
11 F. Mi, X. T. Chen, Y. W. Ma, S. T. Yin, F. L. Yuan and H. Zhang,
Chem. Commun., 2011, 47, 12804–12806.
12 D. Rosario-Amorin, X. Wang, M. Gaboyard, R. Clerac, S. Nlate
and K. Heuze, Chem.–Eur. J., 2009, 15, 12636–12643.
13 B. Karimi and E. Farhangi, Chem.–Eur. J., 2011, 17, 6056–6060.
14 J. M. Yan, X. B. Zhang, T. Akita, M. Haruta and Q. Xu, J. Am.
Chem. Soc., 2010, 132, 5326–5327.
15 X. W. Teng, D. Black, N. J. Watkins, Y. L. Gao and H. Yang,
Nano Lett., 2003, 3, 261–264.
16 (a) M. B. Gawande, R.K. Pandey and R. V. Jayaram, Catal. Sci.
Technol., 2012, 2, 1113–1125; (b) S. U. Sonavane, M.
B. Gawande, S. S. Deshpande, A. Venkataraman and R.
V. Jayaram, Catal. Commun., 2007, 8, 1803–1806.
17 (a) M. B. Gawande, A. K. Rathi, P. S. Branco, I. D. Nogueira,
A. Velhinho, J. J. Shrikhande, U. U. Indulkar, R. V. Jayaram, C.
A. A. Ghumman, N. Bundaleski and O. M. N. D. Teodoro,
Chem.–Eur. J., 2012, 18, 12628–12632; (b) M. B. Gawande,
A. Rathi, P. S. Branco, I. D. Nogueira, C. A. A. Ghumman,
N. Bundaleski and O. M. N. D. Teodoro, ChemPlusChem, 2012,
77, 865–871; (c) M. B. Gawande, P. S. Branco, K. Parghi, J.
J. Shrikhande, R. K. Pandey, C. A. A. Ghumman, N. Bundaleski,
O. Teodoro and R. V. Jayaram, Catal. Sci. Technol., 2011, 1,
1653–1664; (d) M. B. Gawande, A. Velhinho, I. D. Nogueira, C. A.
A. Ghumman, O. M. N. D. Teodoro and P. S. Branco, RSC Adv.,
2012, 2, 6144–6149; (e) M. B. Gawande, S. N. Shelke, A. Rathi, P.
S. Branco and R. K. Pandey, Appl. Organomet. Chem., 2012, 26,
395–400; (f) Y. Z. Chen, D. L. Peng, D. P. Lin and X. H. Luo,
Nanotechnology, 2007, 18, 505703; (g) Y. Z. Chen, X. H. Luo, H.
D. She, G. H. Yue and D. L. Peng, J. Nanosci. Nanotechnol., 2009,
9, 5157–5163; (h) Y. Z. Chen, X. H. Luo, G. H. Yue, X. T. Luo and
D. L. Peng, Mater. Chem. Phys., 2009, 113, 412–416; (i) Y.
Z. Chen, H. D. She, X. H. Luo, G. H. Yue, W. B. Mi, H. L. Bai and
D. L. Peng, J. Nanosci. Nanotechnol., 2010, 10, 3053–3059.
18 H. Z. Guo, Y. Z. Chen, X. Z. Chen, R. T. Wen, G. H. Yue and D.
L. Peng, Nanotechnology, 2011, 22, 195604.
References
1 (a) T. Hirakawa and P. V. Kamat, J. Am. Chem. Soc., 2005, 127,
3928–3934; (b) Z. Li, L. A. Fredin, P. Tewari, S. A. DiBenedetto,
M. T. Lanagan, M. A. Ratner and T. J. Marks, Chem. Mater.,
2010, 22, 5154–5164; (c) N. Toshima, Pure Appl. Chem., 2000, 72,
317–325; (d) Z. K. Wu, H. F. Gian and R. C. Jin, Abstracts of
Papers of the American Chemical Society, 2009, 237; (e) B. G. Choi,
S. Y. Ko, W. Nam and B. Jeong, Bull. Korean Chem. Soc., 2005,
26, 1819–1822; (f) C. J. Zhong and M. M. Maye, Adv. Mater.,
2001, 13, 1507–1511; (g) Y. J. Huang, X. C. Zhou, M. Yin, C.
P. Liu and W. Xing, Chem. Mater., 2010, 22, 5122–5128; (h) J.
X. Wang, H. Inada, L. J. Wu, Y. M. Zhu, Y. M. Choi, P. Liu, W.
P. Zhou and R. R. Adzic, J. Am. Chem. Soc., 2009, 131,
17298–17302; (i) K. Bakhmutsky, N. L. Wieder, M. Cargnello,
B. Galloway, P. Fornasiero and R. J. Gorte, ChemSusChem, 2012,
5, 140–148; (j) S. Alayoglu and B. Eichhorn, J. Am. Chem. Soc.,
2008, 130, 17479–17486; (k) M. Feyen, C. Weidenthaler,
R. Guttel, K. Schlichte, U. Holle, A. H. Lu and F. Schuth,
Chem.–Eur. J., 2011, 17, 598–605; (l) H. L. Jiang, T. Akita and
Q. Xu, Chem. Commun., 2011, 47, 10999–11001; (m) J. H. Liu, X.
F. Wei, Y. L. Yu, J. L. Song, X. Wang, A. Li, X. W. Liu and W.
Q. Deng, Chem. Commun., 2010, 46, 1670–1672; (n) J. Bao, J. He,
Y. Zhang, Y. Yoneyama and N. Tsubaki, Angew. Chem., Int. Ed.,
2008, 47, 353–356; (o) Z. H. Wen, J. Liu and J. H. Li, Adv. Mater.,
2008, 20, 743–747; (p) H. L. Jiang, T. Akita, T. Ishida, M. Haruta
and Q. Xu, J. Am. Chem. Soc., 2011, 133, 1304–1306; (q) R.
G. Chaudhuri and S. Paria, Chem. Rev., 2012, 112, 2373–2433,
and references cited therein.
2 K. Yu, Z. C. Wu, Q. R. Zhao, B. X. Li and Y. Xie, J. Phys. Chem. C,
2008, 112, 2244–2247.
3 S. H. Joo, J. Y. Park, C. K. Tsung, Y. Yamada, P. D. Yang and G.
A. Somorjai, Nat. Mater., 2009, 8, 126–131.
4 V. Mazumder, M. F. Chi, K. L. More and S. H. Sun, J. Am. Chem.
Soc., 2010, 132, 7848–7849.
5 M. M. Maye, J. Luo, S. Chen, W. B. Chan, H. R. Nasland and C.
J. Zhong, Abstracts of Papers of the Am. Chem. Soc., 2003, 226,
U563–U563.
6 C. Koenigsmann, A. C. Santulli, K. P. Gong, M. B. Vukmirovic,
W. P. Zhou, E. Sutter, S. S. Wong and R. R. Adzic, J. Am. Chem.
Soc., 2011, 133, 9783–9795.
7 S. Okada, K. Mori, T. Kamegawa, M. Che and H. Yamashita,
Chem.–Eur. J., 2011, 17, 9047–9051.
19 See supporting information for details.
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